Increasing intelligence conferred on the evolving human brain caused it to struggle with the related metabolic demands, leading to memory disorder.
Alzheimer's disease evolved alongside human intelligence, researchers conclude in a paper posted in May on BioRxi, based on a study of six key genes involved in brain development.
The study found that natural selection drove changes in the six genes which probably helped to increase the connectivity of neurons, making modern humans smarter than hominin ancestors.
But the increased intellectual capacity came at a cost as the same genes are implicated in Alzheimer's disease, reports Nature.
Kun Tang, a population geneticist at the Shanghai Institutes for Biological Sciences in China, who led the research, searched modern human DNA for evidence of this ancient evolution.
They examined the genomes of 90 people with African, Asian or European ancestry, looking for patterns of variation driven by changes in population size and natural selection.
In a control process that was able to separate the two closely resembling effects, they could choose the DNA bits shaped by selection.
Looking back up to 500,000 years ago, from the dawn of modern humans around 200,000 years ago, they arrived at their conclusion.
Most previous methods have only been able to trace back to 30,000 years.
While further studies will confirm the finding, the genetic variation in Asians and Europeans was erased following the migration out of Africa 60,000 years ago. In comparison, the genomes of African people allow researchers to look much further back in time.
Humans are the only species known to develop Alzheimer's and the disease is absent even in closely related chimpanzees.
A neurodegenerative disease with progressive brain cell death, Alzheimer's results in brain size shrinking with time and having progressively fewer nerve cells and connections.
Alzheimer's disease is the most common type of dementia, affecting almost 500,000 people in the UK.